Hydrostatic drives are frequently used as traction drives in mobile working machines. The hydrostatic drives also often have a parking brake, in order to prevent rolling away in the case of a leakage of the hydraulic circuit. In the case of a leakage of this type, hydraulic blocking is not active and a vehicle which is standing, for example, on a hill can roll away. In order to ensure safe parking, as a rule a spring-loaded brake device is used as parking brake which is released during operation by a hydraulic pressure. A system of this type is known, for example, from U.S. Pat. No. 5,884,984. In said document, during traction operation, a piston is loaded counter to the force of a spring by a pressure medium source; this can be, for example, the delivery-side working lines. In order to relieve the pressure space, in which the piston is loaded with the pressure, a connecting line to a tank volume is provided, in which connecting line a throttle point is arranged. A further throttle point is provided between the pressure medium source and the pressure space.
This arrangement has the disadvantage that, in the case of an input-side pressure drop, as can occur, for example, when driving off in traction drives or else when the hydraulic motor outperforms the hydraulic pump, the pressure for opening the brake device can drop, even if possibly only briefly, with the result that an undesired engagement of the parking brake occurs.
Moreover, it is known from U.S. Pat. No. 6,357,558 to connect a pressure space of a brake device of a hydrostatic engine to a cooling circuit via a calibrated non-return valve. The pressure medium which is fed to the pressure space in order to release the brake can be used via the calibrated non-return valve for cooling the brake device which is configured as a multiple disk brake. The actuation of the brake and the interaction with the non-return valve are not described, however.
The disclosure is then based on the object of preventing unintentional activation of the brake device and of it being possible to keep the pressure which is required to release the brake device at as low a level as possible.
The object is achieved by the disclosure by way of the features described herein.
The hydrostatic piston engine according to the engine has a brake device for producing a braking force on a rotor of the piston engine. An actuating element is provided for actuating the brake device, which actuating element, in order to release the brake device, can be loaded with a hydraulic force. Said hydraulic force acts in the form of a pressure medium in a pressure space on a surface of the actuating element. The pressure space is connected to a throttle point in order to reduce the pressure which results from the pressure medium which is made available by the pressure medium source, with the result that the supplied pressure medium can flow from the pressure medium source via the throttle point into the pressure space. Furthermore, the pressure space can be connected to a pressure medium sink, for example the tank volume or the inner housing volume of the hydrostatic piston engine. The pressure level of the pressure medium sink can be assumed to be constant, with the result that a change in the pressure on the side of the pressure medium source results in a corresponding change in the pressure in the pressure space, which pressure acts in the pressure space.
According to the prior art, a further throttle point is provided in the connection between the pressure space and the pressure medium sink. The pressure which is set in the pressure space is therefore dependent on the cross-sectional areas of the two throttle points or nozzles and the pressure of the pressure medium source and of the pressure medium sink. In order to keep the pressure level at a level which is suitable for reliably releasing the brake device without complicated alternative measures, even in the case of a brief drop on the side of the pressure medium source, the pressure level overall has to be relatively high, that is to say, during normal operation, increased considerably in comparison with the minimum pressure for releasing the brake device. In order to avoid this, according to the disclosure, a pressure-holding valve device is then provided between the pressure space and the pressure medium sink in addition to the throttle point which is present upstream of the pressure space in order to reduce the pressure of the pressure medium source to a suitable level for releasing the brake device. Said pressure-holding valve device opens in the direction of the pressure medium sink and has an opening pressure which is at least equal to the pressure in the pressure space which is required to release the brake device. Here, at least equal means that it preferably also lies slightly higher, in order to ensure the release of the brake device even in the case of fluctuations, which are to be taken into consideration, of the parameters of the system, such as pressure medium temperature and therefore viscosity.
The hydrostatic piston engine according to the disclosure has the advantage that, in the case of a pressure drop on the side of the pressure medium source, the non-return valve closes as a result of the pressure-holding valve device before the pressure in the pressure space drops to such an extent that the release of the brake device is no longer ensured.
The description relates to advantageous developments of the hydrostatic piston engine according to the disclosure.